(a) Field of the Technology
The present technology relates to a method for manufacturing a semiconductor substrate and a semiconductor substrate.
(b) Description of Related Art
So far, there have been known SOI (silicon-on-insulator) substrates including an insulating layer and a monocrystalline silicon layer formed on the insulating layer. If elements such as transistors are formed on the SOI substrate, parasitic capacitance decreases and insulation resistance increases. That is, the SOI substrate allows an increase in packing density and an improvement in performance of the elements. The insulating layer is, for example, a silicon oxide (SiO2) film.
Aiming at an increase in operation speed of the elements and a further decrease in parasitic capacitance, the monocrystalline silicon layer in the SOI substrate is desired to be thin. For example, there is a known method for manufacturing the SOI substrate by bonding a pair of substrates (e.g., see Michel Bruel, “Smart-Cut: A New Silicon On Insulator Material Technology Based on Hydrogen Implantation and Wafer Bonding”, Jpn. J. Appl. Phys., vol. 36 (1997) pp. 1636-1641).
Referring to FIGS. 35 to 38, an explanation is given of the method for forming the SOI substrate by the bonding technique. Although there are various techniques for thinning the SOI substrate such as mechanical polishing, chemical polishing or use of porous silicon, hydrogen implantation is employed in this explanation. First, as shown in FIG. 35, the surface of a silicon wafer 101 as a first substrate is oxidized to form a silicon oxide (SiO2) layer 102 as an insulating layer. Then, hydrogen ions are implanted into the silicon wafer 101 through the silicon oxide layer 102 to form a peel layer 104 at a certain depth in the silicon wafer 101 as shown in FIG. 36. Further, the substrate surface is washed by RCA cleaning or the like and a glass substrate 103 as a second substrate is bonded to the surface of the silicon oxide layer 102 as shown in FIG. 37. Then, the obtained structure is heat-treated. At this time, microcracks are generated in the peel layer 104, whereby part of the silicon wafer 101 is separated along the peel layer 104. Thus, the silicon wafer 101 is reduced in thickness. After the separation, the silicon wafer 101 is thinned down to the desired thickness by various techniques such as polishing or etching as required. Further, additional heat treatment is carried out to recover crystal defects caused by the hydrogen implantation or the silicon surface is planarized.
In the above manner, the silicon oxide layer 102 is provided on the surface of the glass substrate (the second substrate) 103 and the silicon wafer 101 which has been reduced in thickness is left on the surface of the silicon oxide layer 102. Thus, the SOI substrate is obtained.
There is also a known method for manufacturing the SOI substrate by implanting hydrogen and boron simultaneously into the silicon wafer 101 (e.g., see G. K. Celler, “Frontiers of silicon-on-insulator”, J. Appl. Phys. Vol. 93 (2003), pp. 4965). The simultaneous implantation of hydrogen and boron is carried out so that the heat treatment for separating part of the silicon wafer 101 along the peel layer 104 can be carried out at a reduced temperature.
However, if hydrogen is implanted into the silicon wafer on which semiconductor elements such as transistors have been formed in advance, the hydrogen ions implanted to peel part of the silicon wafer off may function directly or indirectly as N-type impurities. As a result, adverse effects such as a shift in threshold voltage are caused on the semiconductor elements.
A possible solution of this problem is to heat the SOI substrate to remove hydrogen. However, to completely remove hydrogen from the SOI substrate, the SOI substrate needs to be heated at a temperature as high as about 800° C. or more. Such high temperature environment may change the impurity properties of the semiconductor elements. Therefore, it is substantially impossible to remove hydrogen from the SOI substrate on which the semiconductor elements have been formed. Further, since the second substrate is subjected to a temperature as high as 800° C. or more, the choice of material for the second substrate is limited. For example, glass material having a softening point of about 500 to 700° C. cannot be used.